C-S Bond Cleavage, Redox Reactions, and Dioxygen Activation by Nonheme Dicobalt(II) Complexes

Synthesis and reactivity of a series of thiolate/thiocarboxylate bridged dicobalt(II) complexes were investigated in comparison with their carboxylate bridged analogues bearing free thiol/hydroxyl groups. Upon one-electron oxidation, complexes [Co-2(N-Et-HPTB)(mu-SR1](BF4)(2) (R-1 = Ph, 1a; Et, 1b; Py, 1c) and [Co-2(N=Et-HPTB)(mu-SCOR2)]-(BF4)(2) (R-2 = Ph, 2a; Me, 2b) yielded [Co-2(N-Et-HPTB)(DMF)(2)](BF4)(3) (6) (DMF = dimethylformamide) along with the corresponding disulfides (where N-Et-HPTB is the anion of N,N,N',N'-tetrakis [2-(1-ethylbenzimidazolyl)]2-hydroxy-1,3-diaminopropane). Unlike the inertness of carboxylate bridged complexes [Co-2(N-Et-HPTB)(mu-O2C-R-3-SH)](BF4)(2), (R-3 = Ph, 3a; CH2CH2, 3b) and [Co-2(N-Et-HPTB)(mu-O2CR4)](BF4)(2) (R-4 = Ph, 4a; Me, = 4b; CH2CH2CH2OH, 5) toward O-2, the bridging ethanethiolate in 1b was oxidized to yield a sulfinate bridged complex, [Co-2(N-Et-HPTB)(mu-O2SEt)](BF4)(2) (10). Detailed investigation of the synthetic aspects of 1a-1c led to the discovery of a C-S bond cleavage, reaction and yielded the dicobalt(II) complexes [Co-2(N-Et-HPTB)(SH)(H2O)](BF4)(2) (8a), [Co-2(N-CH2Py-HPTB)(SH)(H2O)](BF4)(2) (8b) (where N-CH2Py-HPTB is the anion of N,N,N',N'-tetralkis[2-(1-picolylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane)), and [Co-2(N-Et-HPTB)(mu-S)(BF4) (9). Both 8a and 8b feature nonheme dinuclear Co(II) units containing a terminal hydrosulfide. The present study thus reports comparative redox reactions for a rare class of 16 dicobalt(II) complexes and introduces a selective synthetic strategy for the synthesis of unprecedented dicobalt(II) complexes featuring only one terminal hydrosulfide.